This invention relates generally to free piston Stirling heat pumps and coolers and more particularly relates to improving the controllability of linear motor driven free-piston Stirling heat pumps and coolers to extract the maximum heat moving capability of the machine in a reliable and repeatable manner but to do so in a manner that avoids damaging collisions of its internal, reciprocating components with its internal stationary components.
Stirling cycle machines have been known for nearly two centuries but in recent decades have been the subject of considerable development because they offer important advantages. Some modern versions are free piston Stirling cycle machines are driven by a linear motor to operate as heat pumps and coolers. However, Stirling heat pumps and Stirling coolers are the same fundamental machines. In both cases the machine has what are often termed a warm end or warm side and a cold end or cold side. The adjectives warmer and colder would be more applicable because they signify the relative temperatures of two spaced apart regions of the machine. The operation of the Stirling machine transfers (“pumps”) heat from the cold end to the warm end. Associated with the cold end is a heat acceptor which transfers heat into the machine. Similarly, associated with the warm end is a heat rejector which transfers heat out of the machine. When a Stirling machine, such as a cryocooler, is used to cool a mass to a temperature below the ambient temperature, it is commonly referred to as a cooler. If a Stirling machine is used to heat a mass above its ambient temperature, such as the air within a room, it is commonly referred to as a heat pump. But in both cases the Stirling machine is pumping heat from its cold end to its warm end so it is cooling its cold end by pumping heat to its warm end. Consequently, as used in the description of this invention, the term cooler includes a heat pump and the term heat pump includes a cooler.
Stirling coolers have a power piston and sometimes a displacer. The power piston is cyclically driven by a prime mover, such as a linear electromagnetic motor. Both the power piston, sometimes referred to as the piston, and a displacer reciprocate within a cylinder. Both the power piston and the displacer function as pistons, the latter cyclically and alternatingly displacing the internal working gas between the warm end and the cold end. Driving the power piston in reciprocation also causes the displacer to reciprocate for reasons well known to those skilled in the art. However, if the linear motor drives these pistons in an excessive stroke (or amplitude which equals one half the stroke), either piston or both can collide with internal components of the Stirling machine. When such collisions have a sufficient impact, the collisions cause severe damage to the internal components.
As is well known in the art, linear motors generally drive free-piston Stirling heat pumps by applying an AC voltage at a particular frequency to the terminals of the linear motor. The amplitude of the piston reciprocation is primarily determined by the amplitude of that voltage. More specifically, the instantaneous amplitude of the piston is directly proportional to the amplitude of the AC voltage applied to the armature of the linear motor. However, the cold-side and warm-side temperatures also influence the amplitude of reciprocation but those temperatures vary at a rate that is much less than several periods of piston reciprocation. Therefore, a Stirling heat pump that is operating without any collisions at one coexisting pair of warm side and cold temperatures and an associated linear motor drive voltage may suffer collisions at the same motor drive voltage when there is a different coexisting pair of warm side and cold side temperatures.
This variation in the motor drive voltage at which collisions occur is important because it is desirable to drive the Stirling heat pump at its maximum amplitude, but without collisions, in order to maximize its efficiency and to maximize its rate of pumping heat. Maximizing its heat pumping rate minimizes the time needed to bring the mass being heated or cooled (the “target”) to the desired temperature. Most heat pump control systems have a set point temperature, which is the desired temperature of the target, and a currently sensed target temperature. Maximizing the heat pumping rate by maximizing the motor drive voltage brings the target temperature to the set point temperature in the minimum time. But damaging collisions must be avoided.
Because the linear motor drive voltage that causes collisions can vary with warm side and cold side temperatures, it is difficult to know what maximum voltage is possible for all operating conditions. This is especially a problem during startup of the machine from ambient temperature at both its cold side and its warm side. During this transient startup, maximizing the heat pumping rate is most desirable but the variations in the cold side and warm side temperatures are greatest because they vary all the way from their initial ambient temperatures to their ultimate steady state temperatures. Current practice often resorts to a slowly ramping voltage applied to the linear motor in order to prevent the machine from being over-stroked at warmer temperatures. But such a slow increase in motor voltage means that the heat pump is operating below, usually far below, its maximum heat pumping capability. Another control possibility determines the piston amplitude from the back EMF of the linear motor and maintains the amplitude below a predetermined amplitude at which collision occurs. Both these techniques suffer from changing temperatures that alter the behavior of the machine. In the first case, due to thermal load, the ramp time may not be sufficient to bring the machine to the required temperatures before full power can be applied. In the second, the temperature effects on the linear motor may alter the relationship between back EMF and amplitude thus reducing the precision of the stroke control. Of course, in any scheme, it is possible to simply allow sufficient safety margins for the piston motions without the possibility of collision by simply driving the pistons at amplitudes that are far below an amplitude that would cause a collision. But this approach results in dead space leading to unused heat transfer rate capacity since the lift or the rate at which is heat moved is dependent on the square of the piston amplitude. In some applications, such as deep temperature freezers, it is important to obtain the fastest cool-down both initially and after door openings in order to limit the exposure of the high value contents of the freezer to extended temperature fluctuations. This can only be achieved by operating the free-piston cooling machine at its maximum possible capacity, which is also the maximum allowable piston amplitude. A further difficulty with current control systems is that, over time, electronic and machine parameters change leading to incorrect control parameters. This can be due to aging or stressing of electrical components or due to gas leakage from the Stirling machine.
An ideal control system for a free-piston, linear alternator driven Stirling engine would therefore have the following attributes:
a. Maximum piston amplitude should be achieved at any combination of cold-side and warm-side temperatures.
b. Self-calibration owing to tolerances in the components of the free-piston Stirling heat pump, linear motor and control electronics.
c. Self-recalibration when the control parameters have changed due to aging, wear or gas leakage and are no longer accurate enough to maintain the machine within its operating envelope.